FutureTask

FutureTask在JDK1.5引入，在IDEA中查看继承关系图：

A FutureTask can be used to wrap a Callable or Runnable object. Because FutureTask implements Runnable, a FutureTask can be submitted to an Executor for execution.

源码中的一段话介绍，FutureTask作为Runnable、Callable的包装类对象，可以被线程池执行或者提交，一般用于线程池获取异步计算结果。
FutureTask也提供了get方法来获得任务执行返回结果。

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    if (s <= COMPLETING)
        s = awaitDone(false, 0L);
    return report(s);
}

get方法是阻塞的，如果当前状态值state<=COMPLETETING就会调用awaitDone方法直到返回结果。FutureTask内部定义了一组任务执行状态

private volatile int state;
private static final int NEW          = 0;
private static final int COMPLETING   = 1;
private static final int NORMAL       = 2;
private static final int EXCEPTIONAL  = 3;
private static final int CANCELLED    = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED  = 6;

其循环过程为：NEW -> COMPLETING -> NORMAL NEW -> COMPLETING -> EXCEPTIONAL NEW -> CANCELED NEW -> INTERRUPTING -> INTERRUPTED。state的默认值为0也就是处于NEW状态。
通过一段代码简单演示FutureTask的使用。

package futuretask;
import java.util.concurrent.*;
public class FutureTaskTest {
    private final static ConcurrentMap<Object, Future<String>> taskCache = new ConcurrentHashMap<>();
    private static String executionTask(String taskName) throws ExecutionException, InterruptedException {
        while (true) {
            Future<String> future = taskCache.get(taskName);
            if (future == null) {
                Callable<String> task = () -> {
                    TimeUnit.SECONDS.sleep(5);
                    return taskName;
                };
                FutureTask<String> futureTask = new FutureTask<>(task);
                future = taskCache.putIfAbsent(taskName, futureTask);
                if (future == null) {
                    future = futureTask;
                    futureTask.run();
                }
            }
            try {
                return future.get();
            } catch (CancellationException e) {
                taskCache.remove(taskName, future);
            }
        }
    }
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        String testTask = executionTask("task");
        System.out.println(testTask);
    }
}